Applications of Derivatives: Chapter 6

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Questions and Answers

If a variable y changes with respect to another variable x, and this relationship is defined by the equation $y = f(x)$, what does the derivative $\frac{dy}{dx}$ represent?

  • The average change in _y_ with respect to _x_.
  • The limit of _y_ as _x_ approaches infinity.
  • The instantaneous rate of change of _y_ with respect to _x_. (correct)
  • The total change in _y_ as _x_ changes.

Given that $x = f(t)$ and $y = g(t)$, where t is another variable, which of the following expresses $\frac{dy}{dx}$?

  • $\frac{dt}{dx} / \frac{dt}{dy}$
  • $\frac{dx}{dt} / \frac{dy}{dt}$
  • $\frac{dy}{dt} / \frac{dx}{dt}$ (correct)
  • $\frac{dy}{dx} / \frac{dt}{dx}$

A circle's area is increasing. If the radius is increasing at a rate of 2 cm/s, what additional information is needed to determine the rate at which the area is increasing?

  • The circle's circumference.
  • The circle's initial area.
  • The circle's current radius. (correct)
  • The rate of change of the circle's diameter.

The volume of a cube is increasing at a constant rate. Which statement accurately describes how the surface area increases?

<p>The surface area increases at a rate inversely proportional to the side length. (D)</p> Signup and view all the answers

A stone dropped into a lake creates circular waves. If the radius of one wave increases at 5 cm/s, which of the following is needed to calculate how quickly the enclosed area is growing?

<p>The wave's current radius. (C)</p> Signup and view all the answers

If the length of a rectangle decreases at 2 cm/min and the width increases at 3 cm/min, how can you determine the rate of change of the rectangle's area?

<p>When both length and width are known. (B)</p> Signup and view all the answers

Given the total cost $C(x)$ for producing x items, what does the marginal cost represent?

<p>The rate of change of total cost with respect to the number of items. (B)</p> Signup and view all the answers

Given the total revenue $R(x)$ from selling x items, what does marginal revenue represent?

<p>The rate of change of total revenue with respect to the number of items. (B)</p> Signup and view all the answers

What does it mean if $f'(x) > 0$ on an interval (a, b)?

<p>The function f(x) is increasing on (a, b). (D)</p> Signup and view all the answers

If $f'(x) < 0$ on an interval (a, b), what can be concluded about the function f on that interval?

<p>The function f is decreasing on (a, b). (C)</p> Signup and view all the answers

A function is said to be constant on an interval I if:

<p>f(x) = c for all x in I, where c is any real number. (A)</p> Signup and view all the answers

What condition must be met for a function f to be increasing on an interval [a, b]?

<p>f(x₁) &lt; f(x₂) whenever x₁ &lt; x₂. (C)</p> Signup and view all the answers

If $f'(x) = 0$ for all x in an interval [a, b], then f is:

<p>Constant on [a, b]. (D)</p> Signup and view all the answers

What does concavity indicate about the rate of change of a function's derivative?

<p>Whether the derivative is increasing or decreasing. (D)</p> Signup and view all the answers

If $f''(x) > 0$ on an interval, what does this imply about $f'(x)$ on the same interval?

<p>$f'(x)$ is increasing. (C)</p> Signup and view all the answers

For an increasing function f, how does $f(x_1)$ relate to $f(x_2)$ if $x_1 < x_2$?

<p>$f(x_1) &lt; f(x_2)$ (C)</p> Signup and view all the answers

What is a necessary condition for f to have a local maximum or minimum at a point c?

<p>f'(c) = 0 or f' is not differentiable at c (D)</p> Signup and view all the answers

What does the First Derivative Test help determine?

<p>Whether a critical point is a local maximum, a local minimum, or neither. (A)</p> Signup and view all the answers

What does it mean if $f'(x)$ changes sign from negative to positive at $x = c$?

<p>There is a local minimum at $x = c$. (A)</p> Signup and view all the answers

What does it mean if $f'(x)$ changes sign from positive to negative at $x = c$?

<p>There is a local maximum at $x = c$. (A)</p> Signup and view all the answers

If $f'(c) = 0$ and $f''(c) < 0$, what does this indicate about the function fat $x = c$?

<p>There is a local maximum at $x = c$. (A)</p> Signup and view all the answers

In the context of optimization problems in calculus, what is an objective function?

<p>A function that is to be maximized or minimized. (D)</p> Signup and view all the answers

What is an absolute maximum value of function f on an interval [a, b]?

<p>It is a value f(c) such that f(c) ≥ f(x) for all x in the interval [a, b]. (A)</p> Signup and view all the answers

Does every function have an absolute maximum and minimum value on its domain?

<p>Only continuous functions on closed intervals have both a maximum and a minimum. (D)</p> Signup and view all the answers

If a function f is monotonic on an interval I, where does f assume its maximum and minimum values?

<p>At the end points of the interval I. (A)</p> Signup and view all the answers

What values do you need to check to find absolute maximum and minimum on a closed interval?

<p>The endpoints of the interval and the critical points of f. (C)</p> Signup and view all the answers

What is the term for a point that is neither a local maximum nor a local minimum?

<p>Inflection point. (D)</p> Signup and view all the answers

Which statements are true about point of inflection?

<p>Point of inflections is where the function changes concavity. (A)</p> Signup and view all the answers

What is the criteria for the second derivative at point of inflections if it exists?

<p>It is will be zero. (A)</p> Signup and view all the answers

What steps would you take to solve an optimization problem?

<p>Draw a diagram, find relevant formulas, create an equation, then take 1st and 2nd derivative. (C)</p> Signup and view all the answers

Which of the options describes correctly where absolute max can be found?

<p>An absolute maximum may occur at any critical point or any endpoint. (A)</p> Signup and view all the answers

What happens to the curve to draw a rectangle with a local minima at A and C if you draw a rectangle?

<p>The function has minimum value in some neighbourhood of A and C which are at the botton of their respective hills. (B)</p> Signup and view all the answers

Flashcards

Derivative ds/dt

Rate of change of distance s with respect to time t.

dy/dx or f'(x)

Represents the rate of change of quantity y with respect to quantity x.

Chain Rule

It says dy/dx = (dy/dt) / (dx/dt) if dx/dt ≠ 0, relating rates of change and variables x, y, and t.

Increasing Function

A function where f(x₁) < f(x₂) when x₁ < x₂.

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Decreasing Function

A function where f(x₁) > f(x₂) when x₁ < x₂.

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Critical Point

A point in the domain of a function f where either f'(c) = 0 or f is not differentiable.

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Local Maxima

A point ‘c’ where f'(x) changes from positive to negative as x increases through c (f'(x) > 0 to the left of c and f'(x) < 0 to the right of c).

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Local Minima

A point ‘c’ where f'(x) changes from negative to positive as x increases through c (f'(x) < 0 to the left of c, and f'(x) > 0 to the right of c).

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Point of Inflection

A point ‘c’ where f'(x) does not change sign, it's neither a local maxima nor a local minima.

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Second Derivative Test (Maxima)

If f'(c) = 0 and f''(c) < 0, ‘c’ is a point of local maxima.

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Second Derivative Test (Minima)

If f'(c) = 0 and f''(c) > 0, ‘c’ is a point of local minima.

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Absolute Maximum Value

The maximum value of f on the entire interval.

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Absolute Minimum Value

The minimum value of f on the entire interval.

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Study Notes

  • Chapter 6 focuses on the application of derivatives in various disciplines
  • Chapter 5 focused on finding derivatives of composite functions, inverse trigonometric functions, implicit functions, exponential functions, and logarithmic functions

Applications of Derivatives

  • Determining rate of change of quantities
  • Finding equations of tangents and normals to a curve at a point
  • Locating turning points on a function's graph to find local maxima or minima
  • Finding intervals where a function is increasing or decreasing
  • Finding approximate values of certain quantities

Rate of Change of Quantities

  • The derivative represents the rate of change of distance with respect to time
  • If a quantity y varies with another quantity x, satisfying y = f(x), then dy/dx (or f'(x)) represents the rate of change of y with respect to x
  • (dy/dx) at x=x₀ (or f'(x₀)) represents the rate of change of y with respect to x at x = x₀
  • If x and y vary with respect to variable t, and x = f(t) and y = g(t), then according to the Chain Rule: dy/dx = (dy/dt) / (dx/dt), if dx/dt ≠ 0

Increasing and Decreasing Functions

  • A function f is increasing on interval I if for any x₁ < x₂ in I, f(x₁) < f(x₂)
  • A function f is decreasing on interval I if for any x₁ < x₂ in I, f(x₁) ≥ f(x₂)
  • A function f is strictly decreasing on interval I if for any x₁ < x₂ in I, f(x₁) > f(x₂)
  • A function f is constant on I if f(x) = c for all x in I, where c is a constant
  • If there exists an open interval I containing x₀ such that f is increasing or decreasing in I, then f is said to be increasing or decreasing at x₀

First Derivative Test

  • The proof of this test requires the Mean Value Theorem
  • If f is continuous on [a, b] and differentiable on (a, b):
  • If f'(x) > 0 for each x in (a, b), then f is increasing in [a, b]
  • If f'(x) < 0 for each x in (a, b), then f is decreasing in [a, b]
  • If f'(x) = 0 for each x in (a, b), then f is a constant function in [a, b]
  • If f'(x) changes sign from positive to negative as x increases through c, then c is a point of local maxima
  • If f'(x) changes sign from negative to positive as x increases through c, then c is a point of local minima
  • If f'(x) does not change sign as x increases through c, then c is neither a point of local maxima nor minima

Maxima and Minima Concepts

  • Points that define where the function reaches its highest or lowest locally are useful in sketching the graph
  • Absolute max and min can be necessary to solving many applied problems
  • An extreme value happens if there exists a point c in I such that f(c) is either a maximum value or a minimum value of f in I
  • The number f(c), in this case, is called an extreme value of f in I and the point c is called an extreme point

Second Derivative Test

  • Let f be a function defined on an interval I, and let c ∈ I which is twice differentiable at c
  • If f'(c) = 0 and f''(c) < 0, then x = c is a point of local maxima
  • If f'(c) = 0 and f''(c) > 0, then x = c is a point of local minima
  • The test fails if f'(c) = 0 and f''(c) = 0

Absolute Maximum and Minimum Value in a Closed Interval

  • A continuous function on interval I=[a, b] has both absolute max and min value that attains at least once in I
  • Let f is a differentiable function in a closed interval I and let c be any interior point in I:
  • f'(c)=0 if f attains its absolute maximum value at c
  • f'(c)=0 if f attains its absolute minimum value at c

Working Rule

  • Find all critical points of f in the interval where f'(x) = 0 or f isn't differentiable
  • Take the end points of the interval
  • Calculate the f values at each of the point listed in step 1 & 2
  • Find the max and min values of f out of the vlaues calculated in step 3

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